Nuclear fuel assembly

ABSTRACT

A fuel assembly for a boiling water reactor which is designed to allow water, during operation of the reactor, to flow upwards through the fuel assembly while absorbing heat from a plurality of fuel rods, whereby part of the water is transformed into steam. The fuel assembly comprises a first steam pipe ( 10   a ) arranged with its longitudinal axis parallel to the longitudinal axis of the fuel assembly and the steam pipe comprises an inlet for the steam arranged in the first end of the steam pipe and an outlet for the steam arranged in the second end of the steam pipe. The fuel assembly also comprises a second steam pipe ( 10   b ) arranged above and at a distance from the first steam pipe such that an opening is formed between the steam pipes. The outlet of the first steam pipe has a diameter which is larger than the diameter of the inlet of the second steam pipe.

TECHNICAL FIELD

The present invention relates to a fuel assembly for a boiling waterreactor which is adapted, during operation of the reactor, to allowcooling water to flow upwards through the fuel assembly while absorbingheat from a plurality of fuel rods, whereby part of the cooling water istransformed into steam, and where the fuel assembly comprises a steampipe through which the steam flows upwards through the fuel assemblytowards the outlet end thereof.

BACKGROUND ART

In a boiling water nuclear reactor, moderated by light water, fuelexists in the form of fuel rods, each one containing a stack of pelletsof a nuclear fuel arranged in a cladding tube. A fuel bundle comprises aplurality of fuel rods arranged in parallel with one another in acertain definite, normally symmetrical, pattern, a so-called lattice,and are retained at the top by a top tie plate and at the bottom by abottom tie plate. A fuel assembly comprises one or more fuel bundles,each one extending along the main part of the length of the fuelassembly and being surrounded by a substantially square fuel channel.

The core is immersed into water which serves both as coolant and asneutron moderator. The fuel assemblies are arranged vertically in thecore and spaced from each other. During operation, the water is admittedthrough the bottom of the fuel assembly and then flows upwards throughthe fuel assembly past the fuel rods. The fuel rods emit heat which istaken up by the water which starts boiling, whereby part of the water istransformed into steam. The water and the steam are passed out throughthe upper end of the fuel assembly.

The produced steam is delivered to turbines which drive generators whereelectrical energy is generated.

A disadvantage with a boiling reactor is the high proportion of steam inthe upper part of the fuel assembly. When the proportion of steam risesin the coolant, its ability to carry off heat from the fuel rods isreduced, thus increasing the risk of dryout, which in turn leads to anincrease of the risk of fuel damage. Still another problem with a highsteam volume in the fuel is that steam is much inferior to water asmoderator, which results in the moderation being insufficient wherebythe fuel is utilized inefficiently. In the lower part of the fuelassembly, the moderator consists of water whereas the moderator in theupper part of the fuel assembly consists of both steam and water. Thismeans that the fuel in the upper part of the fuel assembly cannot beutilized efficiently. It is, therefore, desirable to keep down the steamvolume in the coolant while at the same time maintaining the steamgeneration at a high level.

U.S. Pat. No. 5,091,146 discloses a fuel assembly which attempts toachieve a separation of the steam flow and the water flow in the upperpart of the fuel assembly by arranging a pipe above one or morepart-length fuel rods, that is, fuel rods extending from the bottom tieplate but terminating below and at a distance from the top tie plate.When designing the pipe, there are primarily two problems which have tobe solved, namely, how the steam, which is continuously produced alongthe upper part of the fuel assembly, is to enter the pipe, and also howthe water, which is deposited on the inner walls of the pipe, is to becarried away. These problems have been solved by providing the envelopesurface of the pipe with a number of openings arranged axially atdifferent levels. Some of the openings are formed so as to carry awaythe water which is accumulated on the inner surfaces of the pipe, andother openings are formed for passing the steam into the pipe. Theopenings which are intended for passing in the steam are complicated intheir design and it is doubtful whether the steam can really find itsway through these openings. An additional disadvantage is that theopenings are complicated to manufacture.

SUMMARY OF THE INVENTION

The object of the invention is to produce a fuel assembly with a steampipe, which in an efficient way separates steam from water, which passesthe steam out of the fuel assembly, and which is simple to manufacture.

What characterizes a fuel assembly according to the invention willbecome clear from the appended claims.

A fuel assembly according to the invention comprises a plurality ofsteam pipes arranged one above the other with their longitudinal axesparallel to the longitudinal axis of the fuel assembly. Duringoperation, the fuel assembly is arranged vertically in the core andsteam flows upwards through the steam pipes. The steam pipe has an inletfor the steam in one of its ends, the inlet end, and an outlet for thesteam in its other end, the outlet end. The steam pipes are arrangedspaced from one another such that openings are formed between the steampipes. These openings have two functions; for one thing, steam is toflow into the steam pipes and, for another, the water which is formed onthe insides of the steam pipes is to be passed out. The envelopesurfaces of the steam pipes therefore need not be provided with specialopenings for admission of steam and discharge of water.

To separate the water from the insides of the steam pipe and prevent itfrom following the steam flow into the next steam pipe, the outlet andinlet of the steam pipes are designed such that the internal radius ofthe outlet is larger than the external radius of the inlet. The distancebetween two steam pipes shall be so large that it provides a sufficientinflow area for the steam while at the same time it must not be so largethat the separated water has time to be deflected to such an extent thatit follows the steam up into the next steam pipe. Preferably, theopening between the steam pipes shall have an area which is of the orderof magnitude of near the cross-section area of the steam pipe.

The water on the inside of the steam pipe forms a coherent water film.In one embodiment of the invention, the outlet end of the steam pipe isprovided with means for collecting water from the water film and leadingthe collected water towards the outlet. In this way, large water dropsare formed in localized paths. These large water drops are not deflectedas easily as smaller water droplets, which reduces the risk of the waterbeing brought in with the steam. When the water drops are collected inpaths, almost water-free paths are also formed between the paths withwater drops, and in these water-free paths the steam may flow into thesteam pipe without being obstructed. In this way, the risk of the waterdrops preventing the steam from flowing into the steam pipe is reduced.

To facilitate the inflow of steam to the steam pipe, the oppositelypositioned inlet and outlet ends may be designed tapering towards theopenings for venturi effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a fuel assembly comprising steam pipes according to afirst embodiment of the invention.

FIG. 1b shows a cross section Ib-Ib′, and FIG. 1c shows a cross sectionIc-Ic′ through the fuel assembly in FIG. 1a.

FIG. 2 shows in more detail how an inlet end and an outlet end for asteam pipe may be designed.

FIG. 3 shows another embodiment of the inlet end and the outlet end ofthe steam pipe.

FIG. 4 shows part of a fuel assembly comprising a steam pipe accordingto a second embodiment of the invention.

FIG. 5 shows part of a fuel assembly comprising a steam pipe accordingto a third embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1a shows an embodiment of a fuel assembly according to theinvention. During operation, the fuel assembly is arranged vertically inthe reactor core. FIG. 1b shows a vertical section Ib-Ib′ through thefuel assembly, and FIG. 1c shows a vertical section Ic-Ic′ through thefuel assembly. The fuel assembly comprises an upper handle 1, a lowerend portion 2 and a plurality of fuel units 3 a, 3 b and 3 c stacked ontop of each other. Each fuel unit comprises a plurality of fuel rods 4arranged between a top tie plate 5 and a bottom tie plate 6. The fuelunits are stacked on top of each other in the longitudinal direction ofthe fuel assembly and they are stacked in such a way that the top tieplate 5 in one fuel unit is facing the bottom tie plate 6 in the nextfuel unit in the stack a fuel rod 4 comprises fuel in the form of astack of uranium pellets arranged in a cladding tube 7. The fuelassembly is enclosed in a fuel channel 9 of substantially square crosssection. In this embodiment, the fuel assembly comprises eight fuelunits.

A fuel unit has 100 fuel rod positions in an orthogonal 10×10 lattice. Afuel rod position is a position in the lattice in which it is possibleto arrange a fuel rod. All the positions in the lattice need not beoccupied by fuel rods. The fuel unit is divided into four sub-bundleswith 25 fuel rod positions in an orthogonal 5×5 lattice. The lattice inone sub-bundle comprises a fuel rod position in the centre of thesub-bundle, and around this an inner square ring is arranged consistingof 8 fuel rod positions. Outside the inner ring, there is an outersquare ring consisting of 16 fuel rod positions.

The fuel assembly comprises three different types of fuel units 3 a, 3b, 3 c. In the two lowermost fuel units 3 a, all the fuel rod positionsare occupied by fuel rods which are arranged in parallel with thelongitudinal axis of the fuel assembly. The fuel rods in the fuel units3 b and 3 c are arranged so as to be inclined between the bottom tieplate and the top tie plate. In one sub-bundle, all the fuel rods in thetwo rings are inclined in the same direction, that is, either clockwiseor counterclockwise around the centre of the sub-bundle. The purpose ofinclining the fuel rods around the centre of the sub-bundle is to setwater and steam, which flow upwards through the fuel assembly, intorotation, thus achieving a separation of water and steam. Such a fuelassembly is known from Swedish patent specification No. 96024476.

The fuel unit 3 b has 96 fuel rods distributed among four sub-bundles.Each one of the sub-bundles comprises 24 fuel rods arranged in an innerring 11 a and an outer ring 11 b. The fuel rods are inclined in thedirection of the arrows, that is, around the centre of the sub-bundle.The fuel rod position in the centre of the sub-bundle is unoccupied. Inthis way an empty volume is created in the centre of the fuel bundle.The empty volume constitutes the lower part of a vertical channel whichextends through the six uppermost fuel units in the fuel assembly.

There are four channels 12 a, 12 b, 12 c, 12 d extending through thefuel assembly, one channel in each sub-bundle. The inclined fuel rods inthe sub-bundle bring about an eddy of water and steam around thechannel. The direction of the eddies is marked by arrows in the channel.In this eddy, the water and the steam are separated from each other bythe water being thrown outwards while at the same time the steam ispressed against the centre of the eddy.

The four uppermost fuel units 3 c each have 80 fuel rods distributedamong four sub-bundles. In each sub-bundle, the fuel rod position in thecentre and four positions in the inner ring are unoccupied, thus formingan empty volume which extends through the sub-bundle. The empty volumescontribute to the four channels 12 a-12 d which extend through the fuelassembly. In each one of the channels, four steam pipes 10 a, 10 b, 10 cand 10 d are arranged one above the other such that their longitudinalaxes coincide with one another. Through the steam pipes, the steam isconducted towards the outlet of the fuel assembly. In this embodiment,all the steam pipes are designed identically. Each fuel unit 3 ccomprises four steam pipes arranged between the bottom tie plate and thetop tie plate.

FIG. 2 shows in more detail how a steam pipe may be designed. The lowerend 14 of the steam pipe, hereinafter referred to as the inlet end ofthe steam pipe, has an opening which constitutes an inlet 14 b for thesteam. The upper end 13 of the steam pipe, hereinafter referred to asthe outlet end of the steam pipe, has an opening which constitutes anoutlet 13 b for steam and water which have accumulated on the inner sideof the steam pipe. The outlet end of the first steam pipe 10 a isarranged at a distance from the inlet end of the next steam pipe 10 b.The inlet end of the steam pipe has an outer diameter D2 which issmaller than the diameter D3 of the opening in the outlet end. In thisway, the water 15 emanating from the inside of the first steam pipe isthrown out at a distance from the inlet of the next pipe, thusseparating the water from the steam which continues up through the nextsteam pipe. The distance between two steam pipes shall be so large as toprovide a sufficient inflow area for the steam while at the same time itmust not be so large that the separated water has time to be deflectedto such an extent that it follows the steam up into the next steam pipe.Preferably, the opening between the steam pipes shall have an area whichis of an order of magnitude near the cross-section area of the steampipe, which is determined by the diameter D1 of the steam pipe. Thearrows in the figure show the direction of the steam flow.

The inflow of the steam is facilitated by designing the outlet ends andinlet ends of the steam pipes such that venturi effect is obtained. Forthis purpose, the inlet ends and the outlet ends are arranged such thatthey are tapering towards the opening. The diameter D2 of the inlet andthe diameter D3 of the outlet are to be smaller than the diameter D1 ofthe steam pipe.

The inside of the outlet end 13 is provided with slightly angularlyadjusted grooves 16 which open out into the outlet. The task of thesegrooves is to collect water from the water film 17 which covers theinside of the steam pipe and to concentrate the water to the orifices 16b of the grooves. In this way, large water drops are formed in localizedpaths. One advantage of this is that large water drops are not deflectedas easily as smaller water drops and hence the risk of the wateraccompanying the steam into the next steam pipe is reduced. In addition,the localized paths with water cause formation of water-free pathsbetween these first-mentioned paths, through which steam may flow intothe next steam pipe without being obstructed.

The inlet end 14 is provided with a rejection edge 18 and the outlet end13 is provided with a rejection edge 19 for scraping off the water whichis transported along the outside of the steam pipe. To reinforce theformation of the water film on the inside of the steam pipe, the insideof the steam pipe may be provided with oblique vanes 20. Alternatively,vanes may be arranged in the bottom tie plate 6. The inlet end of thesteam pipe is attached to the bottom tie plate 6 with a plurality ofattachment means 21 and the outlet end of the steam pipe is attached tothe top tie plate 5 with a plurality of attachment means 22.

FIG. 3 shows another example of how the outlet end 25 and the inlet end26 of the steam pipe may be designed. The opening edge of the outlet endis provided with lugs 27. The lugs have the same function as the groovesin the preceding example, namely to form large water drops in localizedpaths. The figure shows the difference between the shape of the waterdrops when the opening edge is provided with lugs 27 and when theopening edge is straight 28. At the straight opening edge, a curtainwith smaller water drops is formed which risk penetrating into theopening between the steam pipes. The steam pipe is provided with arejection ring 29 on its inlet end and with an additional rejection ring30 at its outlet end. The task of the rejection rings is to remove thewater which is accumulated on the outside of the steam pipe. The outletend of the steam pipe is attached to the top tie plate 5 with attachmentmeans 32 and the inlet end of the steam pipe is attached to the bottomtie plate 6 with attachment means 31.

Swedish patent document 9604720-4 shows a fuel assembly with fuel unitsstacked on top of each other, in which the fuel rods are arranged in apolar lattice comprising a number of concentric rings. In such a fuelassembly, the invention may be advantageously applied.

FIG. 4 shows part of a fuel assembly according to a second embodiment ofthe invention. The fuel assembly comprises a number of fuel units 40a-40 c which all have fuel rods arranged in a polar lattice comprisingthree concentric rings. In the lower part of the fuel assembly, fuelunits 40 a are arranged. All the fuel rod positions in the fuel unit 40a are occupied. In the fuel unit 40 b, all the fuel rod positions arealso occupied. The fuel rods 42 have a smaller diameter than the fuelrods 41 in the fuel unit 40 a. The fuel rods in the inner rings areinclined outwards from the centre of the rings so as to form an emptyvolume in the centre. The fuel rods are not inclined in the direction ofthe ring, as is the case in the first embodiment. In this way, no eddyis formed which separates the steam from the water. One advantage ofinclining the fuel rods outwards is that the water accompanies the fuelrods, and in this way a certain separation of water and steam occurs. Inthe fuel units 40 c, the fuel rod positions in the inner ring areunoccupied and, instead, a steam pipe 10 a, 10 b is arranged in thecentre. The steam pipes have the same design as has been describedabove. The advantage of this embodiment is that it is simpler tomanufacture than the first-embodiment.

FIG. 5 shows part of a fuel assembly according to a third embodiment ofthe invention. The fuel assembly comprises a top tie plate 37, a bottomtie plate 38 and a plurality of full-length fuel rods 36 which arearranged between the top tie plate and the bottom tie plate. Further,the fuel assembly comprises a number of part-length fuel rods 39 whichextend from the bottom tie plate and terminate far below the top tieplate. Above these part-length fuel rods, a plurality of steam pipes 10a, 10 b are arranged. The steam pipes are arranged one above the other.Between the steam pipes, spacer elements 44 are arranged to keep thesteam pipes in spaced relationship to each other and to fix the steampipes to each other. To keep the fuel rods in spaced relationship toeach other, a number of spacers 45 are arranged in spaced relationshipto each other along the fuel assembly in the longitudinal directionthereof. The steam pipes are attached to the spacers. The steam pipesare designed in the same way as has been described in the precedingembodiments.

What is claimed is:
 1. A fuel assembly for a boiling water reactor adapted during operation of the reactor to allow coolant to flow upwards through the fuel assembly while absorbing heat from a plurality of fuel rods to transform a portion of the water into steam, said fuel assembly comprising: a first steam pipe arranged with a longitudinal axis parallel to a longitudinal axis of the fuel assembly, wherein the first steam pipe comprises an inlet for the steam arranged in a first end of the first steam pipe, and an outlet for the steam arranged at a second end of the first steam pipe; a second steam pipe arranged above and spaced from the first steam pipe along the longitudinal direction of the fuel assembly such that an opening is formed between the first and second steam pipes, wherein the outlet of the first steam pipe has an outlet diameter which is larger than an inlet diameter of the inlet of the second steam pipe, wherein fuel rods are arranged laterally with respect to said first and second steam pipes in said fuel assembly, wherein a second end of the second steam pipe comprises means for collecting water on an inside of the second steam pipe and conducting the collected water towards the outlet of the second steam pipe.
 2. A fuel assembly according to claim 1, further comprising a third steam pipe arranged above and spaced from the second steam pipe such that an opening is formed between the second and third steam pipes, wherein an outlet of the second steam pipe has a diameter which is larger than a diameter of an inlet of the third steam pipe.
 3. A fuel assembly according to claim 1, wherein the second end of the first steam pipe is arranged tapering towards the outlet of the first steam pipe and the first end of the second steam pipe is arranged tapering towards the inlet of the second steam pipe for achieving a venturi effect.
 4. A fuel assembly according to claim 1, wherein a distance between the outlet of the first steam pipe and the inlet of the second steam pipe is less than half a diameter of the inlet of the second steam pipe.
 5. A fuel assembly according to claim 1, wherein the second end of the first steam pipe comprises means for collecting water on an inside of the first steam pipe and conducting the collected water towards the outlet of the first steam pipe.
 6. A fuel assembly according to claim 5, wherein said means for collecting water comprise a plurality of elongated grooves on the inside of the first steam pipe.
 7. A fuel assembly for a boiling water reactor adapted during operation of the reactor to allow coolant to flow upwards through the fuel assembly while absorbing heat from a plurality of fuel rods to transform a portion of the water into steam, said fuel assembly comprising: a first steam pipe arranged with a longitudinal axis parallel to a longitudinal axis of the fuel assembly, wherein the first steam. pipe comprises an inlet for the steam arranged in a first end of the first steam pipe, and an outlet for the steam arranged at a second end of the first steam pipe; a second steam pipe arranged above and spaced from the fist steam pipe along the longitudinal direction of the fuel assembly such that an opening is formed between the first and second steam pipes, wherein the outlet of the first steam pipe has an outlet diameter which is larger than an inlet diameter of the inlet of the second steam pipe, wherein fuel rods are arranged laterally with respect to said first and second steam pipes in said fuel assembly, wherein the second end of the first steam pipe comprises means for collecting water on an inside of the first steam pipe and conducting the collected water towards the outlet of the first steam pipe, wherein said means for collecting water comprise a plurality of lugs arranged around the outlet of the first steam pipe.
 8. A fuel assembly according to claim 1, wherein the fuel assembly comprises at least two fuel units stacked on top of each other, each fuel unit comprising a top tie plate, a bottom tie plate, a plurality of fuel rods extending between the top tie plate and the bottom tie plate, and one of said first and second steam pipes.
 9. A fuel assembly according to claim 8, wherein the first end of the first steam pipe is attached to the bottom tie plate and the second end of the first steam pipe is attached to the top tie plate.
 10. A fuel assembly for a boiling water reactor adapted during operation of the reactor to allow coolant to flow upwards through the fuel assembly while absorbing heat from a plurality of fuel rods to transform a portion of the water into steam, said fuel assembly comprising: a first steam pipe arranged with a longitudinal axis parallel to a longitudinal axis of the fuel assembly, wherein the first steam pipe comprises an inlet for the steam arranged in a first end of the first steam pipe, and an outlet for the steam arranged at a second end of the first steam pipe; a second steam pipe arranged above and spaced from the first steam pipe along the longitudinal direction of the fuel assembly such that an opening is formed between the first and second steam pipes, wherein the outlet of the first steam pipe has an outlet diameter which is larger than an inlet diameter of the inlet of the second steam pipe, wherein fuel rods are arranged laterally with respect to said first and second steam pipes in said fuel assembly, wherein the second end of the first steam pipe comprises means for collecting water on an inside of the first steam pipe and conducting the collected water towards the outlet of the first steam pipe, wherein a second end of the second steam pipe comprises means for collecting water on an inside of the second steam pipe and conducting the collected water towards the outlet of the second steam pipe. 